1. Field of the Invention
The invention relates to coating formulations comprising particles which on their surface possess isocyanate-reactive groups, and to their use as coating materials.
2. Description of the Related Art
Coating systems comprising particles—more particularly nanoparticles—are state of the art. Such coatings are described for example in EP 1 249 470, WO 03/16370, US 20030194550 or US 20030162015. The particles in these coatings lead to an improvement in the properties of the coatings, and more particularly with regard to their scratch resistance and also, where appropriate, their chemical resistance.
A frequently occurring problem associated with the use of the—generally inorganic—particles in organic coating systems consists in a usually inadequate compatibility between particle and coating-material matrix. This can lead to the particles being insufficiently dispersible in a coating-material matrix. Moreover, even well-dispersed particles may undergo settling in the course of prolonged standing or storage times, with the formation, possibly, of larger aggregates or agglomerates, which even on redispersion are then impossible or difficult to separate into the original particles. The processing of such inhomogeneous systems is extremely difficult in any case, and in fact is often impossible. Coating materials which, once applied and cured, possess smooth surfaces are generally not preparable by this route or are preparable only in accordance with cost-intensive processes.
It is therefore favorable to use particles which on their surface possess organic groups which lead to improved compatibility with the coating-material matrix. In this way the inorganic particle becomes “masked” by an organic shell. Particularly favorable coating-material properties can be achieved in this context if, furthermore, the organic functions on the particle surfaces are also reactive toward the coating-material matrix, so that under the respective curing conditions of the coating material in question they are able to react with the matrix. In this way, success is achieved in incorporating the particles into the matrix chemically in the course of coating-material curing, which often results in particularly good mechanical properties but also an improved chemical resistance. Systems of this kind are described for example in DE 102 47 359 A1, EP 832 947 A or EP 0 872 500 A1. A disadvantage of the systems described there are the generally relatively high levels of the comparatively expensive nanoparticles as a proportion of the coating material's overall solids content.
Also known, furthermore, is the use of coatings which comprise a binder which has been modified with nanoparticles. These coatings can be produced by reacting the particles, equipped with a reactive functionality, with a binder containing a complementary function. In this case, therefore, the organofunctional particle is incorporated chemically into the coating-material matrix not only at the coating-material curing stage but also even at the binder preparation stage. Systems of this kind are described for example in EP 1 187 885 A or WO 01/05897. They possess the disadvantage, however, of being relatively complicated to prepare, leading to high preparation costs.
In the case of one particularly important type of coating material, a film-forming resin is used which comprises hydroxyl-functional prepolymers, and more particularly hydroxyl-functional polyacrylates and/or polyesters, which on curing of the coating material are reacted with an isocyanate-functional curative (polyurethane coating materials) and/or with a melamine curative (melamine coating materials). The polyurethane coating materials are notable for particularly good properties. For instance, polyurethane coating materials possess in particular a superior chemical resistance, while the melamine coating materials generally possess better scratch resistances. These types of coating material are typically used in particularly high-value and demanding fields of application: for example, as clearcoat and/or topcoat materials for OEM paint systems in the automobile and vehicle industry. The majority of topcoat materials for automotive refinish also consist of systems of this kind. The film thicknesses of these coatings are typically situated in ranges from 20 to 50 μm.
In the case of the polyurethane coating systems, a distinction is generally made between what are called the 2K and the 1K systems. The former consist of two components, of which one is composed essentially of the isocyanate curative, while the film-forming resin with its isocyanate-reactive groups is contained in the second component. Both components in this case must be stored and transported separately and should not be mixed until shortly before they are processed, since the pot life of the completed mixture is greatly limited. Often more favorable, therefore, are the 1K systems, which consist of only one component, in which alongside the film-forming resin there is a curative with protected isocyanate groups. 1K coating materials are cured thermally, the protective groups of the isocyanate units being eliminated, and the deprotected isocyanates being able then to react with the film-forming resin. Typical baking temperatures of such 1K coating materials are 120-160° C. Melamine coating materials are generally 1K coating materials; the baking temperatures are typically situated in a comparable temperature range.
In the case of these high-value coating materials in particular, a further improvement in properties would be desirable. This is true more particularly of vehicle finishes. For instance, the attainable scratch resistance of conventional auto finishes, in particular, is still not sufficient, with the consequence, for example, that particles in the wash water in a carwash lead to significant marring of the finish. Over time, this causes lasting damage to the gloss of the finish. In this situation, formulations that allowed better scratch resistances to be achieved would be desirable.
One particularly advantageous way of achieving this object is to use particles having, on their surface, organic functions which are reactive toward the film-forming resin or else toward the curative. Moreover, these organic functions on the particle surface lead to masking of the particles and thus enhance the compatibility between particles and coating-material matrix.
Particles of this kind with suitable organic functions are already known in principle. They and their use in coatings are described for example in EP 0 768 351, EP 0 832 947, EP 0 872 500 or DE 10247359.
The scratch resistance of coatings can in fact be increased significantly through the incorporation of these kind of particles. However, in all of the methods of using these particles that have been described in the prior art, optimum results have still not been achieved. In particular, the corresponding coatings have such high particle contents that on grounds of cost alone it would be difficult to realize the use of such coating materials in large-scale production-line coating systems.
WO 01/09231 describes particle-containing coating systems characterized in that there are more particles located in a surface segment of the coating material than in a bulk segment. An advantage of this particle distribution is the comparatively low particle concentration which is needed for a marked improvement in scratch resistance. The desired high affinity of the particles for the surface of the coating material is achieved by applying a surface-active silicone resin agent to the particle surfaces. The modified particles obtainable in this way possess the relatively low surface energy often typical of silicones. As a consequence of this they arrange themselves preferentially at the surface of the film-forming matrix. A disadvantage of this method, however, is the fact that not only the silicone-resin modification of the particles but also the preparation of the silicone resins themselves that are required for that purpose are costly and complicated from a technical standpoint. A particular problem associated with the preparation of the silicone resins is the fact that the attainment of effective scratch resistance requires the silicone resins to be provided with organic functions, carbinol functions for example, via which the particles thus modified can be incorporated chemically into the coating material when the latter is cured. Silicone resins functionalized in this way are available commercially not at all or only to a very restricted degree. In particular, however, the selection of organic functions that are possible at all in the case of this system is relatively limited. For this system, therefore, as also for all of the other prior-art systems, optimum results have still not been achieved.